The invention is directed to improvements in processes for filtering hot, unsaturated, saturated, or supersaturated slurry, using a vacuum filter without a precoat, especially in filtering a phosphoric acid slurry formed during the manufacture of phosphoric acid and containing typically up to about 15%, more usual 3-9%, by weight of particulate solids.
The invention involves a process for removing particulate impurities from impure aqueous phosphoric acid containing particulate impurities and comprises:
(a) passing the acid at an elevated temperature through a phosphoric acid-resistant fabric filter supported on a porous medium (preferably a rotary drum), to cause at least a portion of the particulate impurities in the acid to be filtered therefrom and to be retained by the fabric filter, thereby forming on the fabric filter a filter cake comprising said particulate impurities and entrained phosphoric acid; and
(b) removing at least a portion of said filter cake comprising particulate impurities from said fabric filter (preferably while said fabric filter is still supported on said porous medium), thereby to present a fresh surface, (i.e., a surface substantially free of filter cake) of said fabric filter for subsequent filtration.
Prior to removing the filter cake it is preferred to contact at least a portion of said filter cake with a displacing liquid to remove a majority of the entrained phosphoric acid and leave a washed filter cake. The amount of the displacing fluid, its pressure, and the angle of application relative to the filter cake are preferably chosen so as to minimize penetration of the solid impurities through the fabric filter into the filtrate.
In the invention, the filter cake is preferably removed by applying a displacing fluid, such as air, steam, water or aqueous phosphoric acid, to the side of the fabric filter opposite the filter cake, thereby driving the filter cake away from the fabric filter. The filter cake can either be a product or a by-product or a waste product.
The bulk of phosphoric acid is produced by the so-called wet process method. Although there are a wide variety of wet process methods, they all basically comprise the acidulation of phosphate bearing rock with sulfuric acid to cause the precipitation of calcium sulfate and the release of crude phosphoric acid. The acid thus produced analyzes from about 22 to about 52 percent P.sub.2 O.sub.5 and the lower P.sub.2 O.sub.5 content phosphoric acids are usually subsequently concentrated by evaporation to so-called merchant grade. The concentrated phosphoric acid is subsequently utilized in the production of fertilizers or is subsequently treated to produce high grade phosphoric acid (such as superphosphoric acid) or other phosphates.
The unpurified phosphoric acid is relatively high in impurities including soluble and insoluble aluminum, magnesium and iron complexes. The insoluble impurities comprise very fine particles which are suspended in the unpurified acid solution, are not normally removed during the acid manufacturing process, nor are they removed by holding the crude acid in a settling tank for a commercially reasonable time, i.e., on the order of 48 to 60 hours. Consequently, the impurities are encountered in the form of a sludge in the bottom of a tank car after shipping the acid from the manufacturing site to the customer's location. The presence of this sludge often results in the rejection of the shipment by the customer or in the loss of a substantial portion of the unpurified acid when decantation of the clear acid is attempted.
The suspended sludge-forming particulate impurities are frequently of a very small particle size (e.g., dominant particle size) and pass through many types of filter media. They also can be gelatinous in nature and can quickly plug up many filter mediums. Prior art methods for purifying the phosphoric acid, such as, for example, filtration through a diatomaceous earth precoat cake, add substantially to the expense of manufacturing the acid due to the increased capital, operating, maintenance costs, loss of P.sub.2 O.sub.5 values in the discarded solids and material costs, e.g., for diatomaceous earth or other filter aid for the precoat. One advantage of the present invention over such prior art methods is that it does not require a precoating of diatomaceous earth or other filter aid on the filter medium; however, it can be sometimes advantageous to include a body filter aid in the phosphoric acid prior to filtration.
It has been found, as disclosed in U.S. Patent application Ser. No. 206,785, filed Nov. 4, 1980 of Smith, et al, that by preheating the acid, followed by vacuum filtration through a filter medium including a diatomite filter cake, sludge forming particulate impurities can be removed in sufficient quantity to substantially eliminate the subsequent formation of sludge during shipping and handling of the phosphoric acid product, or of superphosphoric acid made from the filtered phosphoric acid. Sludge forming impurities can also be removed from phosphoric acid by the processes disclosed in U.S. Pat. Nos. 3,642,439; 4,136,199; 4,243,643 and 4,299,804.
U.S. Pat. No. 3,907,680 discloses a process for purifying a crude aqueous wet process produced phosphoric acid solution which includes passing the solution through a layer or bed of perlite (an aluminosilicate). The patent futher teaches that this purification technique may be accomplished via a vacuum filter. The patent does not disclose the use of a fabric filter without a filter precoat or the use of a water spray to remove entrained phosphoric acid from the filter cake. Other process in which phosphoric acid is filtered to remove solids are found (for example) in U.S. Pat. Nos. 4,164,550, and 3,528,771.
U.S. Pat. No. 4,121,968 to Wells discloses a rotary drum vacuum filter apparatus, useful for dewatering wood pulp slurries, wherein, attached to the trailing edge of the suction box, there is a discharge chamber means for discharging the dewatered cake or web from the flexible porous or foraminous belt of material covering the periphery of the filter. In operation, the belt containing the dewatered pulp cake or web passes from the surface of the drum and over an external perforated (or apertured) member which is connected to a source of pressurized air which passes through the perforations, exits through the perforations, impinges upon and passes through the belt and blows the cake or web therefrom. In the Wells apparatus the pulp cake or web is the product; whereas, in the present case the filtrate is usually the desired product and the cake is usually a waste product, although it can sometimes be used as a by-product, e.g. as a component of a fertilizer or animal feed supplement (as by the processes in U.S. Pat. No. 4,243,643 and in commonly-owned copending U.S. Patent application Ser. No. 107,715 of Mills and Newsom, filed Dec. 27, 1979).
In U.S. Pat. No. 4,121,968, the cake or web of pulp must be removed as a continuous, unbroken sheet in order to convert it into dried pulp or paper. In contrast, in the present process, there is no necessity to maintain a continuous sheet when removing the cake and it is usually preferred to use sufficient discharge fluid pressure as to cause the cake to break up on removal from the fabric filter (or belt).
With the use of rotary vacuum precoat filters, as well as other types of precoat filters, some P.sub.2 O.sub.5 values can be lost in the precoating of filter aid. That is, the precoat cake can entrap and retain P.sub.2 O.sub.5 values in the interstices of the cake. Pending applications Ser. No. 107,715 and U.S. Pat. Nos. 4,235,854; 4,303,524 and 4,313,919 disclose apparatus and processes for recovering such P.sub.2 O.sub.5 values from the filter cake in a precoat filter. These processes and apparatus can be useful in practice of the present invention.